Generally, in timing chains in use for valve drives of internal combustion engines, cam shaft chains in use for camshaft-camshaft drives, and balancer chains, tensioners have been used on the slack side of the chain to take up slack in the chain and to apply tension to the chain.
During operation, a piston of the tensioner presses against the chain to maintain tension in the chain. Also, during operation, when tension in the chain increases due to resonance of a chain span, an excessive shock load from the chain acts on the piston of the tensioner, causing the piston to retract into the housing of the tensioner.
In order to prevent such retraction of the piston, a tensioner device with a ratchet mechanism has been proposed as shown in Japanese patent application publication No. 2002-5250(hereinafter referred to as JP 2002-5250).
The above-mentioned ratchet mechanism, as shown in FIG. 3 of JP 2002-5250, is comprised of a plurality of engagement grooves formed on the outer circumferential surface of the piston, a circlip that engages with an engagement groove, and a guide groove formed in the inner circumferential surface of the piston bore in the vicinity of the opening end of the piston bore of the housing. Each of the engagement grooves is formed of a front side tapered surface and a rear side tapered surface that are connected to each other via a curved surface at the deepest portion of the groove. The guide groove is formed of a first tapered stop that the circlip contacts at the time of the rearward movement of the piston and a second tapered stop that the circlip contacts at the time of the forward movement of the piston. Also, par [0045] of JP 2002-5250 describes that it is preferable to make the front side tapered surface of the engagement groove of the piston generally parallel with the tapered surface of the first stop of the guide groove in order to securely prevent retraction of the piston.
In this case, during operation, when the piston moves in the forward direction the rear side tapered surface of the engagement groove of the piston travels over the circlip with the circlip around the outer circumferential surface of the piston contacted by the second stop of the guide groove, and the movement of the piston in the forward direction is thus permitted. On the other hand, when the piston moves in the rearward direction the circlip around the outer circumferential surface of the piston is sandwiched between the front side tapered surface of the engagement groove of the piston and the first stop of the guide groove, and the movement of the circlip is restricted, preventing the piston from retracting.
In the above-mentioned ratchet mechanism of prior art, when the piston has retracted, the circlip fitted around the outer circumferential surface of the piston impacts the first stop of the guide groove of the housing to cause an impact sound. Recently, there has been a demand in the art for decreasing the impact sound generated at the time of piston retraction.
The present invention is directed to decreasing the impact sound by relieving the impact load generated at the time of retraction of the piston of the tensioner with a ratchet mechanism.